The present invention relates to an automatic buttonholer for a zigzag sewing machine.
The control machanism, particularly, the feed control mechanism, of a conventional buttonholer disclosed in Japanese Patent Publication (TOKU-KAI-SHO) No. 57-64086 has a feed control cam for controlling material feed direction in stitching bar tacks at the opposite ends of a buttonhole and in stitching overcast stitches along the opposite legs of the buttonhole, and a feed regulating cam for regulating the direction and the amount of a feed motion. The feed regulating cam has a cam groove. The inner wall surface of the cam groove serves as a forward feed regulating cam surface by which forward feed amount is reduced according to the height of the cam surface, a central cam surface for zero feed and an outer wall surface serving as backward feed regulating cam surface by which backward feed amount is increased according to the height of the cam surface. The position of a pin attached to a first contact arm is changed between positions corresponding to the inner wall surface, the central cam surface and the outer wall surface of the cam groove by controlling the first contact arm with a feed control cam. A second contact arm having a contact-part contacting the feed control cam is necessary for controlling the position of the pin attached to the first contact arm. This feed control mechanism of the conventional buttonholer further needs many other parts, as parts for pivotally supporting the first and second contact arms, an extension spring having one end connected to the first contact arm and the other end connected to the second contact arm so as to bias the first and second contact arms toward each other, extension springs for rotatably biasing the first and second contact arms and many parts for interlocking these many parts. Accordingly, the feed control mechanism has problems that constructing the mechanism with high accuracy is difficult, the mechanism is inevitably large in size and the mechanism requires high manufacturing cost.